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# DigestInputStream

`public class DigestInputStream`
`` `` `extends FilterInputStream` ``

java.lang.Object
↳ | java.io.InputStream
| ↳ | java.io.FilterInputStream
|  | ↳ | java.security.DigestInputStream

A transparent stream that updates the associated message digest using the bits going through the stream.

To complete the message digest computation, call one of the `digest` methods on the associated message digest after your calls to one of this digest input stream's `read` methods.

It is possible to turn this stream on or off (see `on`). When it is on, a call to one of the `read` methods results in an update on the message digest. But when it is off, the message digest is not updated. The default is for the stream to be on.

Note that digest objects can compute only one digest (see `MessageDigest`), so that in order to compute intermediate digests, a caller should retain a handle onto the digest object, and clone it for each digest to be computed, leaving the orginal digest untouched.

See also:

- `MessageDigest`
- `DigestOutputStream`

## Summary

### Fields
`protected MessageDigest` | `digest` The message digest associated with this stream.

### Inherited fields
From class `java.io.FilterInputStream`
`protected InputStream` | `in` The input stream to be filtered.

### Public constructors
`DigestInputStream(InputStream stream, MessageDigest digest)` Creates a digest input stream, using the specified input stream and message digest.

### Public methods
`MessageDigest` | `getMessageDigest()` Returns the message digest associated with this stream.
`void` | `on(boolean on)` Turns the digest function on or off.
`int` | `read()` Reads a byte, and updates the message digest (if the digest function is on).
`int` | `read(byte[] b, int off, int len)` Reads into a byte array, and updates the message digest (if the digest function is on).
`void` | `setMessageDigest(MessageDigest digest)` Associates the specified message digest with this stream.
`String` | `toString()` Prints a string representation of this digest input stream and its associated message digest object.

### Inherited methods
From class `java.io.FilterInputStream`
`int` | `available()` Returns an estimate of the number of bytes that can be read (or skipped over) from this input stream without blocking by the next caller of a method for this input stream.
`void` | `close()` Closes this input stream and releases any system resources associated with the stream.
`void` | `mark(int readlimit)` Marks the current position in this input stream.
`boolean` | `markSupported()` Tests if this input stream supports the `mark` and `reset` methods.
`int` | `read()` Reads the next byte of data from this input stream.
`int` | `read(byte[] b, int off, int len)` Reads up to `len` bytes of data from this input stream into an array of bytes.
`int` | `read(byte[] b)` Reads up to `byte.length` bytes of data from this input stream into an array of bytes.
`void` | `reset()` Repositions this stream to the position at the time the `mark` method was last called on this input stream.
`long` | `skip(long n)` Skips over and discards `n` bytes of data from the input stream.
From class `java.io.InputStream`
`int` | `available()` Returns an estimate of the number of bytes that can be read (or skipped over) from this input stream without blocking by the next invocation of a method for this input stream.
`void` | `close()` Closes this input stream and releases any system resources associated with the stream.
`void` | `mark(int readlimit)` Marks the current position in this input stream.
`boolean` | `markSupported()` Tests if this input stream supports the `mark` and `reset` methods.
`abstract int` | `read()` Reads the next byte of data from the input stream.
`int` | `read(byte[] b, int off, int len)` Reads up to `len` bytes of data from the input stream into an array of bytes.
`int` | `read(byte[] b)` Reads some number of bytes from the input stream and stores them into the buffer array `b`.
`void` | `reset()` Repositions this stream to the position at the time the `mark` method was last called on this input stream.
`long` | `skip(long n)` Skips over and discards `n` bytes of data from this input stream.
From class `java.lang.Object`
`Object` | `clone()` Creates and returns a copy of this object.
`boolean` | `equals(Object obj)` Indicates whether some other object is "equal to" this one.
`void` | `finalize()` Called by the garbage collector on an object when garbage collection determines that there are no more references to the object.
`final Class<?>` | `getClass()` Returns the runtime class of this `Object`.
`int` | `hashCode()` Returns a hash code value for the object.
`final void` | `notify()` Wakes up a single thread that is waiting on this object's monitor.
`final void` | `notifyAll()` Wakes up all threads that are waiting on this object's monitor.
`String` | `toString()` Returns a string representation of the object.
`final void` | `wait(long timeout, int nanos)` Causes the current thread to wait until another thread invokes the `notify()` method or the `notifyAll()` method for this object, or some other thread interrupts the current thread, or a certain amount of real time has elapsed.
`final void` | `wait(long timeout)` Causes the current thread to wait until either another thread invokes the `notify()` method or the `notifyAll()` method for this object, or a specified amount of time has elapsed.
`final void` | `wait()` Causes the current thread to wait until another thread invokes the `notify()` method or the `notifyAll()` method for this object.
From interface `java.io.Closeable`
`abstract void` | `close()` Closes this stream and releases any system resources associated with it.
From interface `java.lang.AutoCloseable`
`abstract void` | `close()` Closes this resource, relinquishing any underlying resources.

## Fields

### digest

    protected MessageDigest digest

The message digest associated with this stream.

## Public constructors

### DigestInputStream

    public DigestInputStream (InputStream stream,
                    MessageDigest digest)

Creates a digest input stream, using the specified input stream and message digest.

Parameters
`stream` | `InputStream`: the input stream.
`digest` | `MessageDigest`: the message digest to associate with this stream.

## Public methods

### getMessageDigest

    public MessageDigest getMessageDigest ()

Returns the message digest associated with this stream.

Returns
`MessageDigest` | the message digest associated with this stream.

See also:

- `setMessageDigest(java.security.MessageDigest)`

### on

    public void on (boolean on)

Turns the digest function on or off. The default is on. When it is on, a call to one of the `read` methods results in an update on the message digest. But when it is off, the message digest is not updated.

Parameters
`on` | `boolean`: true to turn the digest function on, false to turn it off.

### read

    public int read ()

Reads a byte, and updates the message digest (if the digest function is on). That is, this method reads a byte from the input stream, blocking until the byte is actually read. If the digest function is on (see `on`), this method will then call `update` on the message digest associated with this stream, passing it the byte read.

Returns
`int` | the byte read.

Throws
`IOException` | if an I/O error occurs.

See also:

- `MessageDigest.update(byte)`

### read

    public int read (byte[] b,
                    int off,
                    int len)

Reads into a byte array, and updates the message digest (if the digest function is on). That is, this method reads up to `len` bytes from the input stream into the array `b`, starting at offset `off`. This method blocks until the data is actually read. If the digest function is on (see `on`), this method will then call `update` on the message digest associated with this stream, passing it the data.

Parameters
`b` | `byte`: the array into which the data is read.
`off` | `int`: the starting offset into `b` of where the data should be placed.
`len` | `int`: the maximum number of bytes to be read from the input stream into b, starting at offset `off`.

Returns
`int` | the actual number of bytes read. This is less than `len` if the end of the stream is reached prior to reading `len` bytes. -1 is returned if no bytes were read because the end of the stream had already been reached when the call was made.

Throws
`IOException` | if an I/O error occurs.

See also:

- `MessageDigest.update(byte[], int, int)`

### setMessageDigest

    public void setMessageDigest (MessageDigest digest)

Associates the specified message digest with this stream.

Parameters
`digest` | `MessageDigest`: the message digest to be associated with this stream.

See also:

- `getMessageDigest()`

### toString

    public String toString ()

Prints a string representation of this digest input stream and its associated message digest object.

Returns
`String` | a string representation of the object.

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